1. Field of the Invention
The present invention relates to an apparatus for vaporizing a material at vacuum conditions by means of an arc with a target which includes at least at a part of its surface a material to be vaporized. The discharge is made to proceed in an area where a substantial part of the arc current flows mostly through small spots at the surface of the target. The invention also includes an apparatus for igniting the vaporizing arc and a method of vaporizing material in a vacuum by means of an arc at a target wired as a cathode, which target includes at least at a portion of its surface a material to be vaporized. The arc discharge is performed within an area where a substantial part of the arc flow flows mostly through small spots at the surface of the target. The invention also involves a method of igniting the arc a plasma arc arrangement for depositing coatings onto a substrate by means of an apparatus and an application of the method for a vapor depositing onto substrates by means of a plasma arc.
2. Description of the Prior Art
Methods and apparatuses for a depositing of coatings of materials having a relatively high melting temperature by a depositing of a material which has been vaporized by means of an electric arc are known and disclosed in great variety.
The German published patent application DE-OS 21 36 532 discloses an apparatus in which an electric arc is generated between a jacket shaped anode and a cathode mounted on a cooling bed, whereby the cathode includes on its surface a metal to be vaporized. By means of attaching a screen to the cathode it is sought to prevent the cathode spot produced on the cathode from laterally flashing from the vaporization surface. The basic difficulty of this apparatus is that due to the random unguided straying across of the cathode spot the surface of the cathode is eroded in an irregular manner and, furthermore, due to a local overheating, sputtering is produced continuously. This leads to disturbances on the coating to be produced or in the coating, itself. U.S. Pat. No. 3 625 848 discloses an analogue apparatus whereby the cathode is made of the coating material which is to be vaporized. This apparatus incorporates basically the same drawbacks which are featured by the apparatus according to mentioned DE-OS 21 36 532.
In the apparatus according to the U.S. Pat. No. 4,556,471 the attempt is made to influence by means of a magnetic field or by a mounting of a permanent magnet the arc regarding its straying, to such an extent that the surface of the cathode or the so-called target, is eroded substantially regularly. Thereby the cathode is additionally arranged to be insulated relative to the vacuum chamber. Basically, however, the arc spot still moves uncontrollably and randomly on the cathode wherewith the described drawbacks are eliminated only in part and the danger of sputtering still remains. The U.S. Pat. No. 4,620,913 discloses an apparatus in which the straying or flashing of the spot from the cathode is sought to be prevented by means of a specific arrangement of the cathode. The DE-OS 35 28 677 attempts also by an arranging of arc limiting means to prevent the arc spot from straying, whereby here additionally the spot shall incorporate a controlled movement on the cathode by mounting a source of a magnetic field. Due to the pulsed generation of the magnetic field and depending on its polarity the spot is caused to move toward or away from the magnetic field. This is only a relatively directional, but not a basic guiding of the spot such that--albeit somewhat reduced--the danger of a local overheating and accordingly a sputtering is not completely obviated. Also, no successful realization of such a system has been achieved.
In the DE-OS 33 45 493 a straying of the spot is again prevented by means of arranging a limiting ring at the cathode, but without an influencing of the random movement of the spot of the arc. Also the DE-PS 33 45 442 claims the arranging of a limiting ring of a magnetically permeable material.
In the DE-OS 31 52 131 a magnetic field is generated in the vacuum chamber by means of a so-called solenoid. This magnetic field causes a moving of the spot of the cathode ray. Further, an ignition impulse generator for a continuous igniting of cathode ray spots is forseen, which then move toward a surface due to the magnetic field. The spot is, however, basically not controlled, but rather caused merely to move by the random generator along the surface of the cathode toward the surface.
The CH-PS 657 242 describes how the sputtered or macro particles which have formed are precipitated in the plasma. A so-called plasma guide means and a coaxially mounted electro-magnet are arranged which latter is switched opposite to a focussing solenoid. Accordingly, a specific magnetic field is generated which diverts the plasma beam such that sputtered or macro particles contained in the beam are driven out of the plasma guide means. This diverting, however, causes high losses of material. Furthermore, the diverting mechanism, which is coated much stronger than the substrates must be cleaned continuously, which necessitates a high expenditure and considerable inconvenience. Corresponding matter is claimed in the French disclosure FR-A 2 524 254. The DE-PS 32 34 100 relates basically to the subjects of the two aforementioned documents, whereby additionally modifications of the apparatus are proposed in order to secure a more even coating of a workpiece. These, however, have in no way a bearing on the guiding of the spot on the surface of the cathode.
In the DE-OS 37 31 127 an arc is operated in a pulsed manner, i.e. the temperature of the substance is compared with an upper temperature limit, or the arc is interrupted when exceeding this upper limit. This prevents a local overheating at the surface of the cathode. Sputtering which occurs in spit of this is diverted by means of a magnetic field/screening sheet, i.e. the pulsing represents a quite unsufficient solution. Screening by means of a magnetic field and orifices are, however, uneconomical because they cause high expenditures in maintenance and greatly influence the speed of the coating in a negative manner. The described method is specifically suitable for decorative coatings and less for coatings in the technical field.
In the U.S. Pat. No. 3,673,477 a cathode spot is guided by means of a permanent magnet. The anode is or a ring-like design and the cathode is designed as a disk located parallel and staggered relative to the plain of the disk. A magnet is located behind the cathode, by means of which magnet the cathode spot is moved on the surface of the cathode. Although a reduction of the sputtering is caused by this arrangement, the design must again be judged as being complicated.
In the European specification EP-A 0 284 145 the anode is of an annular shape and the cathode is arranged staggered parallel to the plain of the annulus and designed as a cylinder which is rotatable around its axis. In order to prevent an arbitrary moving of the cathode spot a magnet arrangement is forseen which is displaceably located inside of the cathode cylinder. By means of a rotating of the cylinder and a longitudinal displacing of the magnet, the cathode spot is guided on the surface in order to prevent sputtering. The suggested arrangement encompasses an extremely complicated design usable for practical applications.
Both above described cases have shown, furthermore, that the reduction of sputtering depend very strongly on the material of the cathode, whereby they are quite unsufficient for material which are of interest in practical operations, such as for instance, titanium.